Bill Thompson and Joe Lhota usually aren’t on the same political wavelength.

But a startling thing happened a couple of weeks ago at a Democratic mayoral forum: Thompson charged that property assessments were being “artificially manipulated” by the Bloomberg administration to jack up tax collections.

Lhota, the leading Republican mayoral contender, has said much the same thing.

Thompson dropped his bombshell at the Three Parks Independent Democratic Club during a lengthy attack on higher water rates, parking tickets and inflated rents. As a result, it went unnoticed by the large crowd.

Thompson told The Post he’s convinced the property-tax system has been rigged.

“When you look at years like 2008 and 2009 and valuation was going down, but somehow assessments were going up, there’s something wrong there,” he said. “So I believe the numbers are being manipulated.”

In March, Lhota made a similar charge at a GOP mayoral forum.

He asserted that the administration determines how much it needs to collect from property owners and then massages the numbers to fit.

The administration hit back as hard at Thompson as it did at Lhota earlier.

“Mr. Thompson is flat-out wrong,” said Finance Commissioner David Frankel. “Finance uses industry ‘best practices’ to assess properties in compliance with state law. To suggest otherwise is irresponsible.”

As evidence, Frankel sent along a chart showing that the city took down the market value of Class 1 properties — one-, two- and three-family homes — between 2008 and 2011.

The drop came to nearly 8.5 percent in “one of the most stable markets in the nation,” added an aide.

Bills still went up because the property-tax system is exceedingly complex.

That’s small consolation to befuddled homeowners who can’t figure out how taxes could be increasing when their home values were decreasing.

Officials have been promising to reform the system for years. So far, no one has, possibly because any major changes would almost certainly hit small homeowners hardest.

If either Thompson or Lhota make it to City Hall, they’ll be on the hook to take on that monumental challenge.